Converting CPM to neutron flux

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Joshua Guertler
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Converting CPM to neutron flux

Post by Joshua Guertler »

Greetings,

As a part of a science fair project, I will need to calculate the neutron flux of my fusion reactor. Unfortunately, I only possess a 2" boron trifluoride Model G10-12 A neutron counter which provides readings in counts per minute.

Thus, I was curious how I might go about converting neutron counts per minute to neutrons per second. With this rate of neutron release, I am also curious about how I might go further by converting neutrons per second to the neutron flux detected by the neutron counter, assuming that I have the dimensions of the detector. Thank you.

Sincerely,
Joshua Guertler
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Dennis P Brown
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Re: Converting CPM to neutron flux

Post by Dennis P Brown »

Calculating the flux requires knowing the manufacturer's information for that specific detector's response to neutrons - you need to look that up. Otherwise, one needs a bubble detector to calibrate their mfg. specific neutron detector. Edit your post since I am certain you can convert CPM to CPS. ;)
Joshua Guertler
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Re: Converting CPM to neutron flux

Post by Joshua Guertler »

Greetings,

I believe that you have misinterpreted what I was saying. My detector measures neutrons by giving a readout in counts per minute (as most do). I was curious what a "count" actually translates to in terms of the number of neutrons that a "count" is defined as. I was curious what this quantity was so that I could convert from the ambiguous measurement of 'counts' to an actual number of neutrons. Thank you.

Sincerely,
Joshua Guertler
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Dennis P Brown
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Re: Converting CPM to neutron flux

Post by Dennis P Brown »

One cannot translate counts per time into neutron flux unless one knows what the detectors response, geometry & area, moderator used and its thickness, the energy of the neutrons to name a few issues that are required to know so that a flux can be determined - a general definition of a flux is absolute number of neutrons passing through a cm squared - knowing the absolute neutron number is the key here. That is why some use a bubble detector, determine the absolute rate the fusor is producing neutrons for the various conditions and calibrate their "counter" using that. Just knowing a counts per minute doesn't tell one what the flux is. Others here know more and will, no doubt, post.
Joshua Guertler
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Re: Converting CPM to neutron flux

Post by Joshua Guertler »

Greetings,

How would I be able to convert from CPM to neutrons per minute?

Sincerely,
Joshua Guertler
Jake Wells
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Re: Converting CPM to neutron flux

Post by Jake Wells »

Joshua,

You are not listening to anything Dennis is saying. To convert to n/s you need to know about the detector. A lot of information about this is posted in the FAQ's.
“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”
― Nikola Tesla
Jim Stead
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Re: Converting CPM to neutron flux

Post by Jim Stead »

Joshua, you should take a look at Andrew Seltzman's work with his BF3 detector here: http://www.rtftechnologies.org/physics/ ... tector.htm

I do not see your "G10-12 A" detector listed at the old N. Wood counter lab page:
https://web.archive.org/web/20081011151 ... ecs01.html,
so I wonder if in fact you have the same G-10-2A detector that Andrew described.

If so, be aware that his efforts and results may represent a reasonable expectation for your detector, but it's not something you can use directly (if you are concerned with any semblance of accuracy) due to potential detector variances caused by loss of pressure with age, etc.

As Dennis said, you' ll need a bubble detector, or a calibrated neutron counter like Andrew used, in order to calculate the sensitivity of your particular detector. Without knowing that figure, you cannot properly calculate a neutron flux using it.
Andrew Seltzman
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Re: Converting CPM to neutron flux

Post by Andrew Seltzman »

Hi Joshua,

There's no easy way to calculate the conversion from CPM to neutron flux from the tube sensitivity since it depends on things like energy and moderator geometry. You need to test your neutron counter with a known reference. In my case I calibrated my detector against a known calibrated neutron detector by placing both a known distance from a PuBe neutron source. Later, once I got my fusor working, I calibrated the detector against a bubble dosimeter.

The short answer that you are looking for:
Find someone with a calibrated neutron detector and neutron source and ask for help calibrating yours. Asking at the nuclear engineering department of your nearest major university is a great place to start. You could also order a BTI-PND bubble dosimeter, which is a good thing to have since it's immune to electrical noise.

good luck,
Andrew
Andrew Seltzman
www.rtftechnologies.org
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